Project Summary We aim to identify cellular factors that facilitate integration of avian leukosis virus (ALV) into the host cell genome. In preliminary studies, we have noted interactions of the viral integrase protein with a number of candidate host factors such as the FACT proteins (SSRP1 and SPT16) and nucleolar proteins UBTF and nucleolin. Further experiments will study the direct interactions of purified cellular proteins with viral integrase in vitro. The effect of deleting or over-expressing these proteins on viral integration efficiency will be determined. As controls, we will measure the effect of protein knockout on formation of 2-LTR circles and of reverse transcription products. We also propose to investigate whether these proteins influence the targeting of the provirus to specific sites in the genome or promote the catalytic activity of the integrase. We propose to map integration sites by Illumina sequencing after knockout of specific candidate cellular proteins. To determine the specificity of integration for alpharetroviruses, we will also assay integration of MLV and HIV-1 after knockout of the same cellular proteins. Since previous work has indicated that ALV integrates into intergenic regions preferentially, while MLV and HIV-1 prefer to integrate near or within genes, this work may lead to generation of safer retroviral vectors for gene therapy.